1. Field of the Invention
The present invention relates generally to chemical mechanical planarization (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to a compressible ring suitable for use carriers having active retaining rings.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform CMP operations, including topography planarization, polishing, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistors to define the desired functional devices. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. At each metallization level and/or associated dielectric layer, there is a need to planarize the metal and/or dielectric material. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove over-burden materials, such as copper metallization.
In the prior art, CMP systems typically implement rotary, belt, orbital, or brush stations in which rotating tables (platens), belts, pads, or brushes are used to polish, buff, and scrub one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
In a typical CMP system, a wafer is mounted on a carrier, which rotates to provide uniform and symmetrical material removal. The CMP process is achieved when the exposed surface of the rotating wafer is applied with force against a polishing pad, which moves or rotates in a polishing pad direction. Some CMP processes require that a significant force be used at the time the rotating wafer is being polished by the polishing pad.
Normally, the polishing pads used in the CMP systems are composed of porous or fibrous materials. Depending on the type of the polishing pad used, slurry composed of an aqueous solution containing different types of dispersed abrasive particles such as SiO2, CeO2 or Al2O3, may be applied to the polishing pad, thereby creating an abrasive chemical solution between the polishing pad and the wafer.
FIG. 1A depicts a cross-sectional view of an exemplary prior art CMP system. The CMP system of FIG. 1A depicts a carrier head 100 engaging a wafer 102 utilizing a retaining ring 101. The carrier head 100 is applied against the polishing pad surface 103a of a polishing pad 103 with a force F. As shown, the top surface of the retaining ring 101 is positioned above the front surface of the wafer 102. Thus, while the front surface of the wafer 102 is in contact with the polishing pad surface 103a, the surface of the retaining ring 101 is configured not to come into contact with the polishing pad surface 103a. 
Several problems may be encountered while using a typical prior art CMP system. One recurring problem is called xe2x80x9cedge-effectxe2x80x9d caused by the CMP system polishing the edge of the wafer 102 at a different rate than other regions, thereby creating a non-uniform profile on the surface of the wafer 102. The problems associated with edge-effect can be divided into two distinct categories, namely xe2x80x9cpad rebound effectxe2x80x9d and xe2x80x9cedge burn-off effect.xe2x80x9d FIG. 1B is an enlarged illustration of the pad rebound effect associated with the prior art. The pad rebound effect occurs when the polishing pad surface 103a initially comes into contact with the edge of the wafer 102 causing the polishing pad surface 103 to bounce off the wafer 102. As the moving polishing pad surface 103a shifts under the surface of the wafer 102, the edge of the wafer 102 cuts into the polishing pad 103 at the edge contact zone 104c, causing the polishing pad 103a to bounce off the wafer 102, thereby creating a wave on the polishing pad 103.
Ideally, the polishing pad 103 is configured to be applied to the wafer 102 at a specific uniform pressure. However, the waves created on the polishing pad 103 create a series of low-pressure regions such as edge non-contact zone 104axe2x80x2 and non-contact zone 104a, wherein the removal rate is lower than the average removal rate. Thus, the regions of the wafer 102 which came into contact with the polishing pad surface 103a such as the edge contact zone 104c and a contact zone 104b, are polished more than the other regions. As a result, the CMP processed wafer will tend to show a non-uniform undulating surface profile.
Further illustrated in FIG. 1B is the edge xe2x80x9cburn-off.xe2x80x9d As the polishing pad surface 103a comes into contact with the sharper edge of the wafer 102 at the edge contact zone 104c, the edge of the wafer 102 cuts into the polishing pad 103, thereby creating an area defined as a xe2x80x9chot spot,xe2x80x9d wherein the pressure exerted by the polishing pad 103 is higher than the average polishing pressure. Thus, the polishing pad surface 103a excessively polishes the edge of the wafer 102 and the area around the edge contact zone 104 (i.e., the hot spots). The excessive polishing of the edge of the wafer 102 occurs because a considerable amount of pressure is exerted on the edge of the wafer 102 as a result of the polishing pad surface 103a applying pressure on a small contact area defined as the edge contact zone 104c. As a consequence of the burn-off effect, a substantially higher than the average removal rate is exhibited at the area within about 4 millimeters of the wafer edge area. 102. Moreover, depending on the polisher and the hardware construction, a substantially low removal rate is detected within the edge next lower contact pressure zone 104axe2x80x2, an area between about 3 millimeters to about 20 millimeters of the edge of the wafer 102. Accordingly, as a cumulative result of the edge-effects, an area of about 20 millimeters of the edge of the resulting post CMP wafers sometimes could be rendered unusable, thereby wasting silicon device area.
One way to compensate against edge effects is to use an active retaining ring. An active retaining rings is one that can be controlled so that the under surface of the retaining rings is about even with surface of the wafer being polished. To accomplish this, prior art active retaining rings utilize complex force application mechanisms that apply a reactive force to the retaining ring. These systems commonly use springs, air, or a combination of both, and are coupled to feedback electronics. Based on the feedback, the reactive force, which is commonly in terms of pressure, is fed to the active retaining ring.
Although such systems work relatively well, these systems also suffer in that their complexity makes them difficult to design and implement for symmetric repetitive CMP environments. As is well known, a retaining ring typically is round. As such, a system implementing springs or air must arrange a number of spring or air locations around the retaining ring. In doing so, circumstances will arise where the pressure being applied by one spring or air bladders will not match the pressure being applied by another spring or bladder. This difference can, of course, be attributed to any number of factors. Such factors can include uneven wear on springs, leaks in pneumatics, electronic signal delay, or even improperly entered control variables due to human interaction or programming. Also to fabricate air bladders with uniform thickness and geometry is a very complicated task. All of these factors, although controllable to certain degrees, introduce numerous potential problems to troubleshoot when inappropriate CMP results start appearing in processed wafers.
In view of the foregoing, a need therefore exists in the art for a chemical mechanical polishing system that substantially eliminates damaging edge-effects and their associated removal rate non-uniformities while efficiently facilitates slurry distribution.
Broadly speaking, the present invention fills these needs by providing an active retaining ring support. The active retaining ring support is preferably designed from an elastomeric material that will be applied behind a retaining ring. The elastomeric material, once prepared, is configured to provide a controlled and repeatable level of compressive deflection under the working conditions of the CMP operation. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, chemical mechanical planarization (CMP) system having a polishing pad, a carrier body for holding a wafer, a retaining ring, and an active retaining ring support is disclosed. The active retaining ring is defined by a circular ring having a thickness and a width. The circular ring is defined by an elastomeric material. The circular ring is configured to be placed between the retaining ring and the carrier body. The circular ring has a plurality of voids therein, and the plurality of voids are defined in locations around the circular ring. The circular ring has a compressibility level that is set by the elastomeric material mechanical properties and the plurality of voids.
In another embodiment, an active retaining ring support is disclosed. The active retaining ring support is defined by an annular body that is made from an elastomeric material. The annular body has a plurality of recessed regions, and each of the plurality of recessed regions is spaced apart from respective regions. The annular body is configured to have a maximum compressibility level that is set by the number and size of the plurality of recessed regions. A wafer retaining ring is configured to sit over the annular body. The wafer retaining ring is thus capable applying a force to the annular body when contact is made with a polishing pad. The force is capable of compressing the annular body up to a maximum compressibility level as permitted by the mechanical properties of the material.
In yet another embodiment, a wafer carrier for use in chemical mechanical planarization is disclosed. The wafer carrier includes a carrier body and an annular body that is made from an elastomeric material. The annular body has a plurality of void regions, and each of the plurality of void regions is spaced apart from respective regions. The annular body has a maximum compressibility level that is set by the number and size of the plurality of void regions. An annular body support is also provided. The annular body support is connected to the carrier body and designed to receive the annular body. A wafer retaining ring is configured to mate with the annular body. The wafer retaining ring is capable applying a force to the annular body in response to being applied to a polishing pad. This force is capable of compressing the annular body up to a maximum compressibility level as permitted by the mechanical properties of the material.
In still another embodiment, a method for making an active retaining ring support for use in a chemical mechanical planarization (CMP) carrier head is disclosed. The active retaining ring support is configured to be placed between the carrier head and a retaining ring. The method includes: (a) determining a desired level of compression for a CMP process; (b) generating a mold for an annular ring; filling the mold with an elastomeric material; (c) curing the elastomeric material, thus producing an elastomeric annular ring; and (d) defining holes into the elastomeric annular ring to achieve the desired level of compression, the elastomeric annular ring defining the active retaining ring support.
The advantages of the present invention are numerous. Most notably, the active retaining ring support of the present invention is easy to make, does not require complex electronics, and does not wear as do metallic springs. Furthermore, once the compression level is set by defining holes or voids into the material and by appropriate selection of the material based on its mechanical properties, the compression level does not change over time, as the elastomeric material will naturally want to bounce back to its original uncompressed state so long as it is used within the limits of permanent deformation as set forth by its mechanical properties. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.